This invention relates generally to combustion synthesis, more specifically to self-propagating high-temperature synthesis (SHS) and still more specifically to the synthesis of .alpha.-Si.sub.3 N.sub.4 by combustion synthesis.
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the University of California, for the operation of Lawrence Livermore National Laboratory.
Refractory materials, including ceramics, are used in many applications which have specific requirements such as increased resistance to corrosion, greater tolerance to high temperatures, superior mechanical properties, and special electrical characteristics. Some of these applications include electronic devices, cutting tools, industrial machinery, containers for high temperature reactions and the like.
Combustion synthesis is a process which has been known for quite some time, especially in the Soviet Union but has not been widely applied elsewhere. In combustion synthesis, strong exothermic chemical reactions produce heat that causes combustion waves to propagate spontaneously through the reactants, converting them to the desired products, without requiring the addition of thermal energy from an external source. Some of the attractive and desirable characteristics of combustion synthesis for application to ceramics and refractories include: (1) temperatures in the range of 2000.degree.-3500.degree. C. generated without the addition of external energy; (2) the combustion wave which moves rapidly (1.1-10 cm/sec); (3) the high rate of heating at the combustion front; and (4) the volatilization of cation impurities at the combustion front, which creates products that are purer than the reactants. It would be most desirable, therefore, to have available a combustion synthesis route for the production .alpha.-Si.sub.3 N.sub.4.
U.S. Pat. No. 4,161,512 "Process for Preparing Titanium Carbide" issued July 17, 1979 to A. G. Merzhanov et al, discloses a combustion synthesis route for the preparation of titanium carbide.
U.S. Pat. No. 4,337,463 "Controlled Atmosphere Processing of TiB.sub.2 /Carbon Composites", issued Mar. 22, 1983 to L. A. Joo, describes a process for preparing TiB.sub.2 -carbon composites by mixing titanium boride and carbon, pitch and other reactants, pressing the mixture into the desired shape and heating the shaped article to 2100.degree. C. in a nitrogen atmosphere and a noble gas atmosphere above 2100.degree. C.
A. G. Merzhanov and co-workers of the USSR claimed that they synthesized Si.sub.3 N.sub.4 by a combustion process. A. Merzhanov, "Self-Propagating High Temperature Synthesis", Fizica Khimii Soverm. Problemy, pp 6-45 (1983); A. Merzhanov, "From Academic Idea To Industrial Production", Nauk SSR, vol. 1981, No. 10, pp 30-36 (1981).
Transition metal nitrides (TiN, ZrN, HfN and YN) and composites with aluminum oxide (Al.sub.2 O.sub.3) have been synthesized by the combustion of the metal with sodium azide (NaN.sub.3) which is a solid source of nitrogen. The general reaction is given by equation 1. EQU 3Me+NaN.sub.3 .fwdarw.3MeN (1)
where Me is either Zr, Ti, or Hf metal powder. The combustion is carried out in 1 atmosphere of nitrogen with 100% conversion. These combustion processes are described in U.S. Pat. Nos. 4,446,242 issued May 1, 1984 to J. B. Holt; and 4,459,363 issued July 10, 1984 to J. B. Holt.
Si.sub.3 N.sub.4 is an advanced ceramic material which is important because of its wear, corrosion and thermal shock resistance at high temperatures. It would be useful for applications in the construction of heat engines, heat exchangers, cutting tools, radar windows, high temperature bearings and the like. There are three conventional ways of synthesizing Si.sub.3 N.sub.4. These three methods are illustrated by reactions shown in the following equations: EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 ( 2) EQU 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO (3) EQU 3SiCl.sub.4 -4NH.sub.3 Si.sub.3 N.sub.4 -12 HCl (4)
The first method is the direct nitration of silicon powder in a nitrogen atmosphere. The second is the carbothermic reduction of silica by carbon in a nitrogen atmosphere. Vapor phase reaction of SiCl.sub.4 and NH.sub.4 (ammonia) are shown by equation 4. Some of these methods of the preparation of silicon nitride are exemplified by the following patents:
U.S. Pat. No. 4,117,095 "Method of Making .alpha.-Type Silicon Nitride Powder", issued Sept. 26, 1978 to K. Komeya et al, discloses a method for the preparation of high strength .alpha.-silicon nitride using .alpha.-silicon nitride powder as the starting material and including therein additives such as magnesium and yttrium oxide.
U.S. Pat. No. 4,414,190, "Method of Preparing Silicon Nitride", issued Nov. 8, 1983 to M. Seimiya et al, describes a method of preparing silicon nitride by heating a wet process carbon in the presence of a carbon source, such as a hydrocarbon or solid carbon and a nitrogen source such as nitrogen gas or ammonia.
U.S. Pat. No. 4,590,053 "Method for Producing .alpha.-Form Silicon Nitride Fine Powders", issued May 20, 1986 to T. Hashimoto et al, relates to a method for producing .alpha.-form silicon nitride powder by heat-treating in a nitrogen atmosphere, a mixture of silicon nitride powder, carbon, magnesium or calcium and/or compounds thereof.
U.S. Pat. No. 3,839,541 "Silicon Nitride Products", issued Oct. 1, 1974 to R. J. Lumby et al, describes a process for the preparation of silicon nitride powder with a nitriding atmosphere at elevated temperatures below the melting point of the silicon powder.
U.S. Pat. No. 4,399,115 "Synthesis of Silicon Nitride", issued Aug. 16, 1983 to K. Sato et al, describes a process for synthesizing silicon nitride by reacting a silicon halide and ammonia at a high temperature.
However, none of the above-described methods is completely satisfactory because of incomplete reaction, the presence of carbon, or because of high material costs and high cost of production.
Combustion of silicon powder even with the use of a solid source of nitrogen such as NaN.sub.3 is very unlikely at or near 1 atmosphere of nitrogen pressure normally employed for the reaction. The Si and N.sub.2 combustion reaction does not proceed at low nitrogen pressures (1 atm.) because of the high decomposition pressure of Si.sub.3 N.sub.4. When compared to the dissociation pressure of the transition metal nitrides, it is higher at all temperatures. For example, the decomposition pressure for silicon nitride is approximately 100 atmospheres at 2500.degree. C. The experiments to study the combustion of silicon powder (3 .mu. average diameter), as a function of nitrogen pressure, indicates that the powder will not ignite until a pressure of about 450 atmospheres is reached, and even then there is only partial combustion. Only above approximately 680 atmospheres will the silicon powder completely burn with a 92% yield. The yield may be increased to 96% by the addition of up to 20 wt% of Si.sub.3 N.sub.4 powder to the silicon powder prior to ignition. However, the powder product formed is 88-90% .beta.-phase Si.sub.3 N.sub.4. For sintering purposes, the .alpha.-form is preferable because of enhanced sinterability due to the .alpha.-.beta. phase transition. Also, operating at lower pressures would make the combustion process more economical.
A cost-effective process for the production of .alpha.-Si.sub.3 N.sub.4 powder should, therefore, greatly increase its use in high technology applications.
It is, therefore, an object of the present invention to provide a cost effective method for the preparation of .alpha.-silicon nitride powder.
Another object is to provide a combustion synthesis process for the preparation of .alpha.-silicon nitride.
Yet another object is to synthesize .alpha.-silicon nitride at relatively low nitrogen pressures.
Still another object is to provide pure .alpha.-silicon nitride powder and composites thereof.
Yet another object is to provide .alpha.-silicon nitride in relatively pure form and in high yields.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.